In a wireless communication system of a multiple cell environment using the same frequency, adjacent cell interference is generated in a cell overlapping region. In this case, an interference signal has an influence on a signal of users inside a relevant cell and reduces a demodulation performance of user terminals.
Therefore, the wireless communication system uses two techniques in order to remove adjacent cell interference. First, a reception end of the wireless communication system removes an interference signal from a received signal using an interference removing technique, and then performs demodulation. Next, a transmission end of the wireless communication system can reduce the strength of an interference signal influencing an adjacent cell by removing a beam pattern formed in an interference direction using a transmission beam forming technique. For the transmission end to form a beam, channel information associated with users providing service should be known every moment.
In the case where the wireless communication system uses a Time Division Duplex scheme, a downlink channel and an uplink channel of the Time Division Duplex system have the same channel reciprocity. Therefore, the transmission end performs beamforming for a downlink using channel information estimated through sounding signals received from users.
FIG. 1 illustrates the construction of forming a beam without consideration of interference in a conventional wireless communication system.
Referring to FIG. 1, a terminal1 101 receives service from a base station1 100 in a service area of the base station 100, and a terminal2 111 receives service in a service area of a base station2 200.
At this point, the base station1 100 forms a beam and transmits a signal 120 to the terminal1 101. Here, to form the beam, the base station 100 receives a first sounding signal 121 from the terminal1 101 and should estimate an uplink channel. Depending upon realization, the terminal1 101 may receive a pilot or a preamble of a downlink, estimate a channel, and then feed back the estimated channel to the base station1 100. Similarly, the base station2 110 receives a sounding signal 112 from the terminal2 111, estimates a channel, forms a beam, and then transmits data.
In the case where the base station1 100 forms a beam without consideration of an interference, a signal transmitted by the base station1 100 operates as a large influence 130 on the terminal2 111.
As described above, after estimating a channel (at this point, a channel estimation error may occur) using sounding signals 121 and 131 transmitted from the terminal1 101 and the interference terminal2 111, the base station1 100 controls a signal such that the signal does not reach in the direction of the interference terminal2 110 as much as possible, calculates a beam coefficient that can send signals to the terminal1 101 as much as possible, and forms a beam. The first sounding signal 121 of the terminal1 101 is a signal assigned and transmitted from the base station1 100 currently in service. The second sounding signal 131 of the interference terminal2 110 is a signal that is a sounding signal assigned from the base station2 110 and reaches the base station1 100 currently in service.
Also, referring to FIG. 2, to perform beamforming for interference cancellation, a beam coefficient should be calculated and applied to each antenna. At this point, a time delay occurs.
FIG. 2 illustrates a delay until a base station performs a procedure of calculating a beam coefficient and a procedure of applying a beam coefficient.
Referring to FIG. 2, a base station assigns a sounding signal to a relevant terminal through a downlink frame region during an n frame 200. The terminal transmits the sounding signal to the base station through an uplink frame region during an n+1 frame 210. A beam coefficient is calculated during an n+2 frame 220. A beam can be formed by applying a beam coefficient from an n+3 frame 230. Therefore, a delay of about two frames occurs.
At this point, the time delay may be added to a channel estimation error that uses a sounding signal such that nulling may be directed to a different direction, not an interference direction; or a nulling width is not sufficiently large and, therefore, interference is not removed. Therefore, when a channel between a terminal and a base station changes fast, a conventional method has a limitation that an interference removing effect drops down.